34
Operation
T801-4 Installation and Operation Manual
© Tait Electronics Limited September 2007
signal from the GPS receiver, even if invalid, will be presented on the output
of the T801-4. In some cases, however, the GPS alarms (via DIP switches
3 and 4) have no control over the T801-4 1PPS outputs, because the GPS
receiver itself disables its 1PPS output when the GPS fix is lost. This is
described in more detail in the following section.
GPS Receiver 1PPS
Outputs
The differences between the 1PPS outputs from the GPS receivers used
with the T801-4 are as follows:
■
The Trimble internal GPS receiver always supplies a 1PPS signal, even if
it is invalid.
■
The Dalman external GPS receiver supplies a 1PPS signal only if it is
based on a GPS fix.
■
The QS
2
RSU GPS receiver (Trimble Lassen sQ) always supplies a 1PPS
signal, even if it is invalid.
These differences affect the availability of the T801-4 1PPS outputs and
need to be considered if the 1PPS outputs are used. The configuration of
common T801-4 systems described in
“System Configuration” on page 49
also takes into account these GPS receiver characteristics.
The following table outlines the differences in GPS receiver operation and
the effects on the T801-4.
GPS Receiver
GPS Receiver
1PPS Output
State of DIP
Switch 3 & 4
State of T801-4 1PPS Outputs
When GPS Fix Is Lost
internal Trimble Lassen iQ
always on
off
disabled
(after the fault output delay
a
)
on
left running
(with an invalid 1PPS signal)
b
external Dalman 40742
based on GPS fix
off
disabled
c
on
QS
2
Trimble Lassen sQ
always on
off
RSU with v3.00.00.00 & later firmware:
disabled
≈
28 seconds after GPS fix lost
RSU with earlier firmware:
disabled
on
left running
(with an invalid 1PPS signal)
b
a. Refer to
“Fault Output Delay” on page 33
.
b. The maximum drift of any GPS receiver 1PPS output signal could be quite large when it is not based on a GPS fix,
therefore it should not be used for precise synchronisation.
c. DIP switches 3 and 4 have no effect on the T801-4 1PPS outputs when the Dalman external GPS receiver is used.
